1. Field of the Invention
The present invention relates to an objective lens driving apparatus for use in, for example, an optical disk recording and reproduction apparatus for converging light emitted from a light source such as a semiconductor laser or the like to an optical disk, and recording and reproducing information.
2. Description of the Related Art
In general, optical disk recording and reproduction apparatuses such as, for example, laser disk players and compact disk players record signals to and reproduce signals from a recording layer of an optical disk by emitting a light beam such as a laser light beam or the like from an optical head toward the optical disk and receiving the reflected light or transmitted light from the optical disk with the optical head.
An optical head includes an objective lens for inputting and outputting light. The objective lens is disposed to be opposed to the optical disk. A track of the optical disk is traced by moving the objective lens in a radial direction of the optical disk by an actuator.
The objective lens is also moved up and down by an objective lens driving apparatus in accordance with the upward and downward movements of the recording layer of the optical disk caused by a warp of the optical disk. In this manner, the focusing position of the objective lens is adjusted, the tracking shift caused by the decentration of the optical disk is corrected, and a tilt angle of the objective lens with respect to the optical disk is adjusted.
When an optical axis of a light beam emitted from an optical head is tilted with respect to the recording layer of the optical disk, an optical aberration is generated, which may undesirably lower the level of a reproduction signal or cause offset or crosstalk in a focusing servo driving signal for controlling the focus of the optical head or a tracking servo driving signal for controlling the tracking. Specifically in the case of a recording and reproduction apparatus for a high density optical disk such as a digital video disk, which has been recently developed, it is demanded that the angle of the optical axis of the light beam be maintained at the maximum possible precision since even a slight tilt in the optical axis is problematic. Accordingly, an objective lens driving apparatus is also required to control the tilt of the objective lens with high precision.
FIG. 9 is an isometric view of a conventional objective lens driving apparatus 100, and FIG. 10 is an isometric view of a lens holder 102 of the objective lens driving apparatus 100 shown in FIG. 9.
In FIGS. 9 and 10, a focusing direction Z (vertical direction) matches a direction perpendicular to a recording layer of an optical disk (not shown), a tracking direction X matches a radial direction of the optical disk, and a tangent direction Y is a tangent direction of the optical disk and perpendicular to the focusing direction Z and the tracking direction X.
As shown in FIG. 9, the objective lens driving apparatus 100 includes the lens holder 102. As shown in FIGS. 9 and 10, an objective lens 103 is mounted on a center part of a top surface of the lens holder 102. A focusing coil 106 is wound around the lens holder 102, i.e., around the focusing direction Z. In FIG. 10 and the figures described below, parts of the coil 106 on four side surfaces of the lens holder 102 are indicated by reference numerals 106a, 106b, 106a and 106c (see FIG. 11A for the part 106c). A pair of tracking coils 107 are provided in two opposed side surfaces 102a of the lens holder 102 and wound around the tracking direction X. The objective lens 103 is positioned between the pair of tracking coils 107.
As best shown in FIG. 10, four elastic supporting members 105 are connected to the lens holder 102 at one end thereof. Referring to FIG. 9, the four elastic supporting members 105 each pass through respective holes (not shown) in a supporting holder 104 and are connected to a printed circuit board 111 at the other end thereof.
The supporting holder 104 is secured to a securing base 101, and the printed circuit board 111 is secured to the supporting holder 104. Thus, the securing base 101, the supporting holder 104, and the printed circuit board 111 are integrated together. As best shown in FIG. 10, the four elastic supporting members 105 cantilever the lens holder 102 to the securing base 101 to be movable in the focusing direction Z and the tracking direction X.
Returning to FIG. 9, a pair of magnets 108a and 108b have a magnetization direction in the tangent direction Y. The magnets 108a and 108b are provided on the securing base 101 so that the same magnetic pole surfaces thereof face each other. The lens holder 102 is positioned between the magnets 108a and 108b. A pair of magnetic shielding plates 109 each formed of a magnetic material are provided on the securing base 101 so as to interpose the lens holder 102. The magnetic shielding plates 109 are each arranged perpendicular to the tracking direction X.
With reference to FIGS. 11A, 11B and 1C, an operation of the conventional objective lens driving apparatus 100 shown in FIGS. 9 and 10 will be described. FIGS. 11A, 11B and 11C are schematic plan views of the lens holder 102 shown in FIG. 9 and the vicinity thereof.
Referring to FIG. 11A, when an electric current flows in the focusing coil 106 located in the magnetic fields of the magnets 108a and 108b, a force acts on the focusing coil 106 in the focusing direction Z, thereby moving the lens holder 102 in the focusing direction Z. At this point, as shown in FIG. 10, the direction of a force Fa acting on the two opposed parts 106a of the focusing coil 106 and the direction of forces Fb and Fc acting on the other two parts 106b and 106c of the focusing coil 106 are opposite from each other. However, since the parts 106a are closer to the magnets 108a and 108b than the parts 106b and 106c, the number of magnetic fluxes crossing each of the parts 106a is larger than the number of magnetic fluxes crossing each of the parts 106b and 106c. Accordingly, the force Fa is stronger than the force Fb or Fc. As a result, the lens holder 102 moves in the direction of the force Fa.
Referring to FIG. 11B, two opposed parts of each tracking coil 107 are indicated by reference numeral 107a, and the other two opposed parts of each tracking coil 107 are indicated by reference numeral 107b. When an electric current flows in the tracking coils 107 located in the magnetic fields of the magnets 108a and 108b, a force acts on the tracking coil 107 in the tracking direction X, thereby moving the lens holder 102 in the tracking direction X. At this point, the direction of a force acting on the two opposed parts 107a of the tracking coil 107 and the direction of a force acting on the other two parts 107b of the tracking coil 107 are opposite from each other. However, since the parts 107a are closer to the magnets 108a and 108b than the parts 107b, the force acting on the parts 107a is stronger than the force acting on the parts 107b. As a result, the lens holder 102 moves in the direction of the force acting on the parts 107a.
In the state where the lens holder 102 has not been moved in the tracking direction X as shown in FIG. 11A, the force Fb and the force Fc respectively acting on the parts 106b and 106c of the focusing coil 106 have an equal magnitude. Thus, even when the lens holder 102 is moved in the focusing direction Z, the lens holder 102 does not tilt.
However, in the state where the lens holder 102 has been moved in the tracking direction X as shown in FIG. 11C, the angle of magnetic flux Bb crossing the part 106b of the focusing coil 106 (enlarged part D) and the angle of magnetic flux Bc crossing the part 106c of the focusing coil 106 (enlarged part E) are different from each other. Accordingly, the magnitude of vector component Byb, of the magnetic flux Bb, perpendicular to the part 106b, and the magnitude of vector component Byc, of the magnetic flux Bc, perpendicular to the part 106c, are different from each other. That is, .vertline.Byb.vertline.&lt;.vertline.Byc.vertline.. Accordingly, when an electric current flows in the focusing coil 106 in this state, the magnitude of the force Fb is different from the magnitude of the force Fc; that is Fb&lt;Fc. The magnitude difference between the forces Fb and Fc, i.e., Fc-Fb causes the lens holder 102 to rotate around the tangent direction Y. As a result, the objective lens 103 tilts.
The longer the moving distance of the focusing coil 106 in the tracking direction X is, the larger the angular difference between the magnetic flux Bb crossing the part 106b and the magnetic flux Bc crossing the part 106c is.
Such a tilt of the objective lens 103 is caused by the difference between the angle of the magnetic flux Bb with respect to the part 106b and the angle of the magnetic flux Be with respect to the part 106c. The difference between the angles is caused because the magnetic fluxes Bb and Bc are significantly curved in an area extending from the magnets 108a and 108b to the magnetic shielding plate 109.
In order to avoid such a tilt of the objective lens, various proposals have been made. For example, according to the technology disclosed by Japanese Laid-Open Publication No. 7-240031, a top end of a counter yoke is arranged to be at a higher level than a top end of a magnet, so that a moment in proportion to the moving distance of the lens holder in the focusing and tracking directions is generated in the lens holder, and thus a moment of the lens holder caused by the shift between the center of gravity and the supporting center/driving center is always counteracted, regardless of the moving direction and distance of the lens holder. In this manner, the tilt of the objective lens is avoided, and the optical aberration and focal shift are restricted, so as to realize correct recording and reproduction of information.
As described above, in the case of the objective lens driving apparatus 100 shown in FIG. 9, when the lens holder 102 is moved in the focusing direction Z in the state where the lens holder 102 has been moved in the tracking direction X, the force Fb acting on the part 106b of the focusing coil 106 and the force Fc acting on the part 106c of the focusing coil 106 have different magnitudes, thereby tilting the lens holder 102. The longer the moving distance of the focusing coil 106 in the tracking direction X is, the more significantly the objective lens 103 tilts.
As a result, an optical aberration is generated at a spot on the recording layer of the optical disk where the light is converged; a focal shift is generated with respect to the recording layer to inhibit accurate recording of the signals to the recording layer; or signals reproduced from the recording layer is deteriorated.
According to the technology disclosed by Japanese Laid-Open Publication No. 7-240031, the counter yoke is inserted throughout the lens holder. Such a structure increases the size and weight of the lens holder. When the weight of the lens holder is increased; the sensitivity to the acceleration of the lens holder which is required to comply with the fluctuation of the recording layer or decentration of the optical disk; or an excessive load is applied to the contact face between the coil and the lens holder, resulting in deterioration of the transfer efficiency of the force from the coil to the lens holder. Such deterioration of the transfer efficiency lowers the frequency response characteristics of the objective lens.
The technology disclosed by the above-described document has another problem in that since the counter yoke and a rear yoke are connected to each other at bottom portions thereof and still the top ends of the counter yoke and the rear yoke are maintained at a higher level than the magnets. Therefore, the lens holder is enlarged in the focusing direction and is unlikely to be smaller and thinner in the focusing direction.